A structural body which is likely to have an impact is provided with an impact-absorbing structure so as not to transfer an impact to an object to be protected, if the structure should be impacted. Structures for absorbing an impact in the manner that has been described above include movable bodies such as aircraft and automobiles. The aircraft include those having rotating blades such as a helicopter and an airplane. For example, a helicopter is provided with an under-floor structure with an impact absorbing structure for securing the safety of crews on forced landings. In this case, in order to secure the safety of crews in a helicopter, it is desirable to suppress a maximum value of acceleration on a floor surface and also decrease an initial gradient of acceleration on a floor surface immediately after an impact so as to fall within an acceleration profile guaranteed by impact resistant seats on which the crews are seated as shown in FIG. 17. In order to realize the desired acceleration profile, there have been proposed various structures in which impact absorbing members constituting an impact absorbing structure will collapse in a stepwise manner.
To be more specific, there has been proposed an impact absorbing member constituted with a tubular-shaped body which is formed with a fiber-reinforced resin and undergoes a stepwise change in thickness along a central axis thereof (for example, refer to Japanese Patent No. 3141570). In the above-described impact absorbing member, upon impact, collapse occurs in a stepwise manner from a site lower in thickness. Further, as another example, there has been proposed an impact absorbing member which is composed of a resin and a fiber layered body in which needling and stitching are provided in a stepwise manner (for example, refer to Japanese Patent No. 4247038). Upon impact, the above-described impact absorbing member that will collapse in a stepwise manner at a site where the needling and the stitching are provided and that is able to absorb an impact.
However, in the impact absorbing member disclosed in Japanese Patent No. 3141570, at individual sections different in thickness along the central axis, fracture characteristics are different depending on the thickness. Thereby, it is necessary to carry out a test for confirming the fracture characteristics at each of the sections. Further, where a load level, that is, the ratio of load at the start of collapse by an impact to load at completion of the collapse varies widely, there is a case that it is difficult to change the thickness in a stepwise manner to an extent that corresponds to a maximum load in any given length dimension. These problems are also found in the structure disclosed in Japanese Patent No. 4247038 where the needling and stitching are provided. In this case, a confirmation test is needed at every site where the needling and the stitching are provided. There is also a case where it is difficult to provide the needling and the stitching in a stepwise manner to such an extent that corresponds to a maximum load.